Spark ignition engines can have improved operation when operated with a fuel that provides a sufficient ignition delay so that the start of combustion is substantially controlled by the introduction of a spark into the combustion chamber. Fuels that do not have a sufficient ignition delay for an engine can cause “knocking” in the engine, where at least part of the combustion in the engine is not dependent on the introduction of the spark into the combustion chamber.
Traditionally, fuels for spark ignition engines have been characterized based on use of octane ratings. A common method for characterizing the octane rating of a fuel is to use an average of the Research Octane Number (RON) and the Motor Octane Number (MON) for a composition. (RON+MON/2). This type of octane rating can be used to determine the likelihood of “knocking” behavior when operating a conventional spark ignition engine.
Another type of characterization of a fuel for a spark ignition engine is the sensitivity of the fuel, which is defined as (RON−MON). Some previous methods for selecting fuels with longer ignition delays at a given value of RON have involved selecting fuels with lower values of the sensitivity.